1865
J. Am. Chem. Soc. 1984, 106, 1865-1866
thexylchloroborane-methyl sulfide.I6 In our view, these results clearly establish that the hydroboration of alkenes with BH3-Lewis base complexes must involve a prior dissociation of the complex followed by reaction of the free borane with the alkene. As mentioned time and again,I0 kinetic studies of the reaction of BH,-Lewis base complexes with simple alkenes such as 1-octene have been practically impossible due to the complexity of the reaction. However, the reaction of BH3.SMe2 with 2,3-dimethyl-2-butene (TME) stops at the first stage (RBH,), making possible a rigorous kinetic analysis of the reaction. The rate equation for the dissociation mechanism (eq 3 and 4) derived B H 3 - SMe2
&
B H 3 t SMe2
(3)
*-I
(4)
by steady-state treatment predicts complex kinetic behavior. The direct-attack mechanism, on the other hand, requires clean second-order kinetics to be exhibited. Our kinetic analysis of the rate data for the reaction of TME (0.200 M) with BH3-SMe2 (0.200 M) in toluene at 0 "C yielded second-order rate constants decreasing in magnitude with the progress of the reaction, as predicted by eq 5 (Table 111). When the reaction is done in the dp _ dt
--
Asymmetric Addition to Chiral Naphthyloxazolines. A Facile Route to 1,1,2-Trisubstituted-1,2-dihydronaphthalenesin High Enantiomeric Excess Bruce A. Barner' and A. I. Meyers* Department of Chemistry, Colorado State Uniuersity Fort Collins, Colorado 80523 Receiued December 9, 1983 Revised Manuscript Received January 31, 1984 The increasingly important role of asymmetric synthesis has been manifested by the large number of reports on this subject over the past 10 years., Prominent among these studies has been the use of chiral oxazolines as auxiliaries for a wide range of enantiomerically enriched compound^.^ We now describe a novel asymmetric route to chiral 1,1,2-trisubstituted-l,2-dihydronaphthalenes with very high enantio~electivity.~The process involves the nucleophilic addition of various organolithium reagents to the chiral 1-naphthyloxazoline l 5followed by trapping of the intermediate azaenolate with several electrophiles to furnish the dihydronaphthalene 2. This tandem alkylation sequence affords
k,k,[BMS][TME] k-,[SMe2] + k2[TME]
presence of excess Me,S, k-, [SMe,] >> k2[TME], simplifying the rate equation to eq 6.17 Since Me,S is in excess, its condp _ dt
k,k,[BMS] [TME]
k-l w e 2 1
(6)
2 (a-e)
3 (a-e)
2 with a high degree of diastereofacial selectivity thus incorporating centration will be fairly constant, leading to reasonable pseudotwo asymmetric centers in a one-pot reaction.6 Furthermore, a second-order kinetic behavior. We observe this behavior. For mild, high-yield procedure for removal of the oxazoline moiety the reaction of TME (0.200 M) with BMS (0.200 M) in the is described, producing enantiomerically pure 1,2-dihydropresence of excess Me,S (0.400 M) in toluene at 0 "C, good naphthalene aldehydes 3. second-order rate constants are observed (Table 111). Treatment of a T H F solution containing (+)-1 (-45 "C) with It is interesting to note that these results can explain Pasto's an organolithium reagent followed by additin of an electrophile results on the reaction of TME with BH3.THF in T H F at 0 OC.I2 (-45 "C) produced the adducts 2a-e as a mixture of diastereomers, Since the reaction was done in the presence of a large excess of whose ratios were readily assessed by HPLC analysis (Table I). T H F (solvent), k-, [THF] will be very large and thus be constant. In each example, the two diastereomers formed were the result A pseudo-second-orderkinetics will obtain, as was indeed observed. of sequential trans a d d i t i ~ nthus , ~ the diastereomeric ratios reflect (An expression similar to eq 6 should be applicable.) only the facial selectivity of the initial lithium nucleophile. The Thus, our results on the reaction of 2,3-dimethyl-2-butene with absolute configuration as well as the trans addition were confirmed BH3-SMe2provide strong evidence for the dissociation mechanism. by X-ray diffraction studies on pure 2c. Since the oxazoline is We also wish to draw attention to the important observation known to contain the 4S,5S configuration,* the organolithium made by Klein and co-workers.'* They noted that the hydroenters mainly at the @-facefollowed by electrophile entry at the boration of aged solutions of m-methoxystyrene with BH3-THF a-face of the naphthalene ring. The diastereomers of 2 were easily exhibited a considerable induction period. They attributed this separated by flash chromatography providing enantiomerically induction period to the diversion of a reactive intermediate by an pure aldehydes 3 after removal of the oxazoline (vide infra). impurity, probably peroxide. Only after all of the impurity had reacted would the hydroboration itself begin. This experiment clearly shows that the complex, BH3.THF, is not the hydroborating (1) National Research Service Award Postdoctoral Fellow (NIH-IFspecies involved in the actual hydroboration step. 32CA07333). Thus, our present studies and that of Klein indicate that the (21 A comprehensive review on asymmetric synthesis has been compiled: Morrison, J. D. 'Asymmetric Synthesis"; Academic Press: New York, 1983; mechanism of hydroboration of alkenes with BH,-Lewis base Vol. 1-4, in press. complexes proceeds via a prior dissociation of the complex. They (3) Meyers, A. I.; Lutomski, K. A. In "Asymmetric Synthesis"; Morrison, do not support the direct-attack mechanism proposed by Schleyer J. D., Ed.; Academic Press: New York, 1984; Vol. 3, Part 11, in press. and co-workers on the basis of ab initio calculation^.'^ (4) Aryl borate anions have been shown to furnish trans-l,2-disubstituted-1,2-dihydronaphthalenes(Negishi, E.; Merrill, R. E. Chem. Commun. The rate studies were performed by monitoring the disap1974, 860. pearance of the B-H stretching absorbance of the BH3-Lewis base (5) Prepared in 62% yield from 1-naphthoic acid following the procedure complex (-4.0-4.3 j" using a quantiative IR p r o c e d ~ r e . ~ ~ ~reported previously: Meyers, A. I.; Lutomski, K. A. J. Am. Chem. Soc. 1982, (16) Brown, H . C.; Sikorski, J . A,; Kulkarni, S. U.; Lee, H. D. J . Org. Chem. 1982, 47, 863. (17) The term kG,[TME]in eq 6 may still contribute to a minor extent, since a sufficiently large excess of Me,S to eliminate the competition would have altered the medium. As a result, ihe observed pseudo-second-order rate constants kp in Table 111 are not strictly inversely proportional to [Me$]. (18) Klein, J.; Dunkelblum, E.; Wolff, M. A. J . Organomet. Chem. 1967, 226, 57.
0002-7863/84/1506-1865$01.50/0
104, 879. For other derivatives of aryloxazolines, see: Meyers, A. I.; Elanagan, M. A,; Trefonas, L. M.; Baker, R. J. Tetrahedron 1983, 39, 1991. [ c x ] *of ~ ~1 53.0' (c 7.50, CHCI,). ( 6 ) Organolithium reagents add to simple, achiral naphthalene oxazolines and may be trapped to trans-l , I ,2-trisubstituteddihydronaphthalenes(confirmed by X-ray analysis). Lutomski, K. A,, unpublished results. (7) The trans alignment of R to E in 2c as well as the absolute configuration are given with the X-ray details in the supplementary material. (8) Meyers, A. I.; Smith, R. K.; Whitten, C. E. J . Org. Chem. 1979, 44. 2250.
0 1984 American Chemical Society
1866 J . Am. Chem. Soc., Vol. 106, No. 6, 1984 Tablc I. Asymmetric Addition t o 1-Naphthyloxazolines -_____ - compd RLi" ( T , "CJ E b ( T ,O C ) ____--
Communications to the Editor
_______
8;.I:"ii;" 2
3 (%)
~ _ _diast _ _ratiod __---
(%)C
[ o ]2',(CHCl,)e
n m
a
!i-BuLi (-45)
94:6
MeCO,C1(-45) -
/
(99)
(88) Me
b
PhLi (-45)
(99)
MeLi (-45)
[ C H , ) , S L i (-78)
+201'
((
3 60)
+747'
((
4 35)
(88)
Ox
PhS
PhSSPh (-45)
86:14
(5 6) d
CHO
83.17
Me1 (-45)
PhS
C
,f
H
/
CHO
a:' (62)
Me1 (-78)
40 60
+634- (( 0 5 8 )
e
-638" (d
-I-
e, 69)
a
T
M
(
